I. Field of the Invention
The present invention relates broadly to solar heaters which convert incident solar radiation into heat energy and transfer the absorbed heat either to a gas such as air or a liquid such as water, the former being commonly referred to as a solar air heater and the latter being commonly referred to as a solar water heater. More particularly, the present invention relates to solar heaters of either type which employ a heat trap between the absorber and the light-transmitting front wall.
II. Description of the Prior Art
Various proposals have already been made in the prior art for employing a heat trap between the absorber and the front wall of a flat plate solar heater in order to reduce heat losses by natural convection and radiation. An example of the foregoing wherein a transparent honeycomb heat trap is used may be found in an article by Hollands entitled, "Honeycomb Devices in Flat Plate Solar Collectors", Solar Energy, Vol. 9, pp. 159-169, Pergamon Press (1965). One problem encountered with such proposals is that it may as a practical matter be necessary to provide space between the honeycomb heat trap and the front wall to allow for differential thermal expansion of the solar heater elements. It has been found, however, that the presence of such a space can increase the heat loss by natural convection to the front wall. This increased heat loss results from the development of natural convection currents between adjacent cells of the honeycomb structure.
It has been observed in the results of recent work by Edwards, et al., in "End-Clearance Effects on Rectangular-Honeycomb Solar Collectors" presented at the 1975 International Solar Energy Congress and Exposition, July 28-Aug. 1, 1975, UCLA, Los Angeles; also published in Solar Energy, Vol. 18, pp. 253-257, Pergamon Press (1976), that this increase in heat loss may be reduced by providing a space between the honeycomb heat trap and the flat plate absorber rather than between the heat trap and the front wall. The difficulty with this proposal, however, is that the honeycomb must of necessity be made with relatively thin walls in order to act as an efficient heat trap and, consequently, the honeycomb is either too flexible to be self-supporting if made of clear plastic or extremely fragile if made of glass and, therefore, separate means for supporting the honeycomb structure over its entire area must be provided.
It has also been proposed in the prior art to employ a honeycomb or other open cellular structure in contact with the front wall to act as a radiation trap and air buffer in a transpiration solar air heater. Thus, in our copending application Ser. No. 824,100, now U.S. Pat. No. 4,262,657 filed on even date herewith and assigned to the common assignee hereof, and which is a Continuation-in-Part of our earlier application Ser. No. 712,139 filed on Aug. 6, 1976, now abandoned, there is disclosed and claimed a transpiration solar air heater wherein the cellular radiation trap is held in at least firm mechanical contact with the front wall in order that it may additionally act as an air buffer layer. It has been demonstrated by experimental work, as reported in our copending application, that a significant improvement in thermal efficiency is attained when the radiation trap is held firmly in contact with or is actually bonded to the front wall. When the radiation trap, on the other hand, is only loosely held in place adjacent to the front wall, the problem arises that convection currents may pass through the heat trap and contact the front wall where heat losses may occur.
In the case of both the flat plate solar heater and the transpiration solar air heater described above, the heat trap may be supported in contact with the front wall by one or more rigid bars below the cellular or honeycomb heat trap or by the provision of adhesive joints between the heat trap and the front wall. Although this type of support means effectively solves the problem of convective air flow through the heat trap, the drawback of support means of this type is that the bars or adhesive joints or other support elements introduce additional surfaces from which incident solar rays may be reflected or scattered in a direction away from the solar absorber and be lost.